1. Field of the Invention
The present invention relates to control devices for motors and more particularly to control devices for brushless motors which are applied to motor vehicles. More specifically, the present invention is concerned with a control device for accurately controlling the rotation speed of the brushless motor in accordance with an instruction signal applied thereto.
2. Description of the Prior Art
For driving blower fans of automotive air conditioners and cooling fans for radiators of motor vehicles, brushless motors have been widely used. For controlling the brushless motors, a control device including a microcomputer is usually employed.
In order to clarify the task of the present invention, one automotive air conditioner having a brushless motor controlled by a known control device will be described before making a detailed explanation of the present invention.
FIG. 7 is a block diagram of the known automotive air conditioner which employs a brushless motor 4 for driving a blower fan 4a. Denoted by numeral 2 is an air conditioning control unit for centrally controlling the air conditioner. Denoted by numeral 3 is a control panel for permitting a driver or passenger to input instructions to the control unit 2 for setting the temperature in a vehicle cabin. In the illustrated case, the control panel 3 has a power switch and a temperature control knob. Denoted by numeral 1 is a control device for controlling the motor 4. The control device 1 comprises a processing unit 12, a drive section 13 which includes a driver circuit and resistors, and six field effect transistors (MOSFET) 14 each two being applied to one winding of the motor 4. Denoted by numeral 5 is a battery and 6 is a switch circuit for feeding when ON an electric power of the battery 5 to the motor 4. The switch circuit 6 is actuated by an ignition switch (not shown). In the following, a voltage of the electric power supplied from the battery 5 through the switch circuit 6 will be referred to as "VCC voltage".
The control panel 3 provides the air conditioning control unit 2 with an information signal representing the position of the temperature control knob, that is, a temperature desired by the passenger. In addition, information signals issued from various sensors are fed t the control unit 2, which are signals representing the temperature in the vehicle cabin, outside temperature, quantity of solar radiation and so on. Based on the information signals received, the control unit 2 issues to the processing unit 12 a voltage signal representing a desired rotation speed of the motor 4. Because the blower fan 4a is driven by the motor 4, the rotation speed of the motor 4 controls the air amount blown from an air outlet to the vehicle cabin. By means of an analogue/digital converter (A/D converter), the voltage signal from the control unit 2 is converted to a digital data and led into the processing unit 12 for calculation of a target rotation speed value of the motor 4. The target rotation speed value calculated by the processing unit 12 is a data representing a duty (which will be referred to as "input duty" (Din). The data is subjected to an after-described correction and finally becomes a pulse signal duty used for driving the motor 4. Thus, controlling of the brushless motor 4 is carried out by allowing the air conditioning control unit 2 to change the target rotation speed value (viz., input duty) led into the control device 1.
In order to allow the brushless motor 4 to run accurately at the target rotation speed, it is necessary to precisely control the rotation speed of the motor 4 in response to the input duty (Din) led into the control device 1. For dealing with a fluctuation of the VCC voltage caused by a temperature change of the battery 5, a so-called "proportional control" is used for controlling the motor 4. That is, the voltage appears at both ends of each winding of the motor 4 when each field effect transistors 14 is ON, and the voltage fluctuation tends to make a fluctuation of current in the windings. In the proportional control, the input duty (Din) is corrected based on a deviation between the sensed VCC voltage and a reference voltage. This correction will be referred to as "VCC voltage correction" hereinafter.
In the processing unit 12, the input duty (Din) is derived, and the VCC voltage is subjected to sampling to obtain VCC voltage value "Vi". And, in a VCC correction factor calculating section 12a of the processing unit 12, a VCC correction factor "Ma" is calculated by using the following equation (1). EQU VCC correction factor "Ma"=Vt/Vi (1)
wherein:
Vt: reference VCC voltage (=12.5V) stored in a reference voltage generating section 12b.
The VCC correction factor "Ma" is led into a VCC correction section 12c of the processing unit 12, and by using the following equation (2), a VCC voltage correction is carried out in the section 12c to obtain a medium duty (Dm). EQU medium duty =input duty (Din).times.Ma (2)
After correcting the VCC voltage, a linearity correction is carried out to cause the rotation speed of the motor 4 to have a linearity to the medium duty (Dm). That is, when the medium duty (Dm) is led into a linearity correcting section 12e of the processing unit 12, a linearity correction factor "Mc" corresponding to the medium duty (Dm) is looked up from a linearity correction table 12d, and by using the following equation (3), an output duty (Dout) of pulse signal directed to the drive section 13 is calculated. EQU output duty (Dout)=Dm.times.Mc (3)
Upon receiving the pulse signal with the output duty through the drive section 13, the six field effect transistors 14 are turned ON in given order thereby to feed the windings of the motor 4 to drive the same. Accordingly, in the processing unit 12, the voltage signal of the target rotation speed value for the motor 4 is processed for controlling the rotation speed of the motor 4. With this, the rotation speed of the brushless motor 4, that is, the amount of air blown from the air outlet is controlled to a target amount for contributing to the air conditioning in the vehicle cabin.
As is described hereinabove, in the known control device for the brushless motor employed in the automotive air conditioner, in order to drive the brushless motor 4 at a target rotation speed, both a proportional control based on the deviation of the power voltage and a correction for improving the linearity of the rotation speed of the motor 4 are practically carried out.
Recently, with increased demand for amenity in the vehicle cabin, it has become necessary to control the temperature in the vehicle cabin with a much higher accuracy and thus, the control for the brushless motor of a blower fan has needed a higher accuracy.
However, the known control device fails to provide the brushless motor 4 with finely controlled rotation due to a mechanical energy loss of the motor 4 and a ripple on the source voltage of the motor 4. In fact, the dispersion is inevitably produced in accordance with the rotation speed of the motor 4. For solving the drawback, various measures have been proposed. However, almost all have failed to exhibit a satisfied result.